The subject matter of this invention relates generally to circuit breakers with tripping circuits and it specifically relates to circuit breakers with ground fault trip control circuits.
U.S. Pat. No. 3,818,275, "Circuit Interrupter Including Improved Trip Circuit Using Current Transformers", issued June 18, 1974 to A. B. Shimp and assigned to the assignee of the present invention discloses a circuit interrupter for sensing fault current in a line to be protected for deriving intelligence about the status of the fault current and for causing the circuit interrupter to initiate a tripping action when the fault current reaches a predetermined level. After the tripping action has been initiated, sufficient energy for continuing the tripping operation to its completion is then derived from the fault current. Consequently the control system provides intelligence concerning the proper time to trip the circuit breaker prior to the actual tripping operation but does not provide significant tripping energy during this time span as it is obviously not needed. After the tripping operation has begun, however, the control system then provides significant tripping energy but little or no intelligence data, as intelligence data is no longer needed. Sometimes, though it is desirous to sense the presence of ground fault current in a line to be protected and to cause a tripping action or operation based on the presence of the ground fault current. Since ground fault current may not be of sufficient magnitude to supply tripping energy during that part of the tripping operation when it is needed, a separate energy deriving circuit using only ground fault current is provided for substantially completing the ground fault tripping operation once it has begun. The circuit of this type may be found in U.S. Pat. No. 3,959,695 entitled "Circuit Interrupter With Ground Fault Trip Control" issued May 25, 1976 to A. B. Shimp and also assigned to the assignee of the present invention. Sometimes it is desirous to use a summing transformer ground fault sensing system. In that case the three-phase lines, for an example, of a three-phase electrical system are each monitored by a separate current transformer. The output of the current transformer is provided in each case to the input of the normal fault current detecting circuit and in series circuit relationship to a primary winding of the summing transformer, the secondary winding of which is connected to the ground fault sensing circuit. It is well known that the sum of the currents flowing in a polyphase electrical system is generally equal to zero. If ground fault current flows, the latter sum will not equal zero. If the sum of the three-phase currents is not equal to zero, a voltage output will exist on the aforementioned summing transformer secondary winding. This is indicative of the presence of ground fault current. Unfortunately, in an arrangement of the type previously described, the serially connected primary of each primary winding of the summing transformer has a voltage imposed thereacross. This voltage also appears across the current transformer secondary and this requires an exciting current to flow. The exciting current subtracts vectorially from the secondary current that would otherwise normally flow. One way to improve the accuracy of the circuit is to reduce the exciting current relative to the secondary current of the current transformer. This can be done by keeping the voltage across the primary winding of the summing transformer as low as possible. However, it is necessary to provide a minimum amount of voltage at the secondary winding of the summing transformer to operate that portion of the control circuit which is associated with detecting ground faults. In order to keep the primary voltage relatively low while keeping the secondary voltage of the summing transformer sufficiently high to meet the latter mentioned criteria the turns ratio of the summing transformer (from primary winding to secondary winding) must be relatively high. However, if the turns ratio of the summing transformer is relatively high the secondary current thereof tends to be relatively low. It may be so low, that when a ground fault is sensed the magnitude of the secondary current of the summing transformer which is utilized to actuate the trip coil will be insufficient to accomplish the latter purpose. It would be desirous therefore if a ground fault circuit interrupter could be found in which the serially connected primary of the summing transformer for ground fault detection had a relatively low voltage impressed thereacross during normal operation, but which can nevertheless cooperate with the remaining portion of the ground fault detecting circuit for providing sufficient energy to trip the circuit breaker in a reasonable period of time after the ground fault has been detected.